I accidentally hit some combination of CTRL/ALT etc. keys, but I don’t know which ones.
This caused VSCode to switch my editor window to a split screen.

On the left hand side it is now displaying commit history associated with each line in the editor. As I select lines, different blocks of test are highlighted (a cobalt background, but your scheme may be different). I’m sure this mode is helpful if you’re working on the file history, but that is not the problem working on right now and it’s just lost screen real estate and visual noise.

Most importantly, how can I close this pane? Second, how did I open it in the first place?